Hi,
I understand the rationale of this suggestion, but I have a few objections that need to be borne in mind.
First of all, reopening this world to the community will cause a huge disparity between v5 and v6 players: some players amassed a huge fortune over the last version, it wouldn't be fair for the newer players that did not have the opportunity to garner the aforementioned fortune. It would be unfair for us to widen the disparity between the newer and older players of version 6, I'm sure you would agree.
If implemented this would cause huge issues for our wild chunk system, for example a player could very easily claim another player's land and grief/steal all of the items. Furthermore there is no objective way to determine whether a player does, in fact own the land and blocks, due to the fact that we have no block logs in that world.
Finally, this would possibly cause a huge issue in the economy; we know that there is a negative correlation between the number of items in a market and the prices of said items (i.e. the more items in the economy, the lesser they sell for) either from the Auction or Noobstown Shop. This in turn would make it substantially harder to make money, thus it will make it harder for newer players and prospective town owners.
Overall, I don't think that this idea is feasible, but I do understand the personal reasoning of your suggestion.
Hope this helps.
-Waterworth